linux-kernel - Re: [PATCH] mm/writeback: fix dereferencing NULL mapping->host

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Date:   Tue, 29 Nov 2022 09:56:20 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Andrew Yang <andrew.yang@...iatek.com>
Cc:     Masami Hiramatsu <mhiramat@...nel.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Casper Li <casper.li@...iatek.com>,
        <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-mediatek@...ts.infradead.org>
Subject: Re: [PATCH] mm/writeback: fix dereferencing NULL mapping->host

On Tue, 29 Nov 2022 11:32:59 +0800
Andrew Yang <andrew.yang@...iatek.com> wrote:

> From: "andrew.yang" <andrew.yang@...iatek.com>
> 
> Check before dereferencing mapping->host
> 
> Signed-off-by: andrew.yang <andrew.yang@...iatek.com>
> ---
>  include/trace/events/writeback.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
> index 86b2a82da546..56f6e114d3ed 100644
> --- a/include/trace/events/writeback.h
> +++ b/include/trace/events/writeback.h
> @@ -68,7 +68,7 @@ DECLARE_EVENT_CLASS(writeback_folio_template,
>  		strscpy_pad(__entry->name,
>  			    bdi_dev_name(mapping ? inode_to_bdi(mapping->host) :
>  					 NULL), 32);
> -		__entry->ino = mapping ? mapping->host->i_ino : 0;
> +		__entry->ino = mapping && mapping->host ? mapping->host->i_ino : 0;

I hate remembering precedence. Can we add parenthesis around this to be
clear?

		__entry->ino = (mapping && mapping->host) ? mapping->host->i_ino : 0;

Thanks,

-- Steve


>  		__entry->index = folio->index;
>  	),
>